Viruses consist of either double-stranded or single-stranded DNA or RNA enclosed in a protein coat, called a capsid. Some viruses also possess a lipoprotein envelope that, like the capsid, may contain antigenic proteins. Since viruses have no metabolic machinery of their own, they usurp the machinery of their host cell which, depending on the virus, may be a plant, bacterium, or animal cell.
A viral infection begins when a virion comes into contact with a host cell and attaches, or adsorbs, to it. The viral DNA or RNA then crosses the plasma membrane into the cytoplasm and eventually enter into the nucleus. In the case of retrovirus, the viral RNA is reverse transcribed into DNA. Viral DNA is then integrated into the chromosomal DNA of the infected cell. Integration is mediated by an integration protein, integrase. All integrated proviruses are required for the subsequent transcription process which is acted upon by the host cell transcription factors. The integrated DNA is transcribed by the cell's own machinery into mRNA, or replicated and becomes enclosed in a virion. For retrovirus, the integrated DNA is transcribed into RNA that either acts as mRNA or become enclosed in a virion. This completes the virus life cycle.
In the past decade, the emergence of human immunodeficiency virus type 1 (HIV-1) as an important human pathogen has led to a resurgence of scientific interest in retroviruses. HIV-1 is the primary etiologic agent of AIDS, a fatal disease that results from the gradual destruction of the helper T-cell population in infected individuals. The importance of HIV-1 as a human pathogen has led to its being the major focus of research into lentivirus replication and gene regulation. Indeed, HIV-1 may be viewed as the prototype of not only the lentivirus subgroup but also, more broadly, complex retroviruses in general.
There are an estimated 650,000 to 900,000 people currently living with HIV in the United States, with approximately 40,000 new HIV infections occurring here every year. As of June 1999, 711,344 AIDS cases have been reported in the United States. Since the beginning of the epidemic, 420,201 AIDS deaths have been reported. The scale of the AIDS epidemic demands the development of efficient and affordable AIDS therapeutics.
While HIV-1 relies heavily on the cellular host enzymes for many of the steps required in its replication, the virus carries in its genome the genetic information that leads to the synthesis of its unique retroviral enzymes, such as the three enzymes encoded by its pol gene: reverse transcriptase, proteases, and integrase. Effective antiviral agents must inhibit virus-specific replicative events or preferentially inhibit virus-directed rather than host cell-directed nucleic acid or protein synthesis. To date, of the numerous compounds that have already been identified and approved for marketing by the FDA for HIV, only drugs inhibiting the activities of reverse transcriptase and protease inhibitors have been identified. The first drug to be introduced was suramin, a reverse transcriptase inhibitor. Subsequently, AZT and other compounds (zalciabine (ddC), didanosine (ddl), compound Q, ritonavir, etc.) have also been found to possess anti-HIV activity in vitro. Specifically, AZT was approved by the FDA in 1987.
Even though the current therapeutic agents are effective in inhibiting the enzymatic activity which is essential for the viral life cycle, the small fraction of remaining viruses unfortunately mutate and continue to replicate even in the presence of these drugs. High rates of replication, viral sequence mutation, and rapid turnover of the viral population are typical traits of retroviruses. These traits are even more striking in the case of HIV-1. As result, these drugs show little long term benefits in terms of a complete treatment or prevention of HIV-infection. Recent studies have demonstrated that combinatorial therapy against reverse transcriptase (RT) and protease can eliminate a majority of the HIV viruses in T lymphocytes. There is, therefore, need for additional therapeutic agents to be added to the treatment cocktail for viral infections, particularly retroviral infections.
The viral integrase catalyses the integration of the viral DNA into the host DNA, which is an essential step in the viral life cycle. There is no know human homologue to this enzyme and therefore potential inhibitors could be both efficacious and non-toxic. However, drugs targeting integrase have been slow to emerge because of the lack of structural information on this poorly soluble protein. Current search on integrase inhibitor has relied more on empirical testing than on drug design.
Salvia miltiorrhiza is a traditional Chinese medicinal herb for treatment of cardiovascular and hepatic diseases. Extracts from S. miltiorrhiza and its related species exhibit anti-viral and antioxidant activities that are health beneficial. See Meng et al. (1992) Chung Kuo Chung Hsi I Chieh Ho Tsa Chih 12, 345–347, 324–35; Xiong (1993) Chung Kuo Chung Hsi I Chieh Ho Tsa Chih 13, 33–35, 516–517; U.S. Pat. No. 5,178,865; U.S. Pat. No. 5,411,733; U.S. Pat. No. 6,043,276; International PCT Application 98/24460; Chinese Patent Application Nos. 1,192,922 and 1,192,918. Antiviral agents active against herpes, polio, measles, varicellazoster, cytomegalovirus, DNA viruses and RNA viruses have been described which contain at least one crude drug from the root of S. miltiorrhiza Bunge (See European Patent No. 0 568 001 A2). Seven phenolic compounds isolated from the aqueous extract of S. miltiorrhiza demonstrate a strong protective action against peroxidative damage to liver microsomes, hepatocytes, or erythrocytes (See Liu, et al., 1992, Biochem. Pharmacol. 43, 147–1952). Lithospermic acid B was identified as an active component in an extract of Salvia miltiorrhiza radix that was shown to exhibit endothelium-dependent vasodilation in the aorta and may be useful in the treatment of hypertension (See Kamata, et al., 1993, Gen. Pharmacol. 24, 977–981). The therapeutic effect of these extract has been attributed in part to the ability of the plant to accumulate active compounds such as transhinones and phenolic compounds.
Therefore, there remains a need in the art for the identification of additional compounds capable of treating viral infections, particularly compounds that inhibit viral integrase.